The function of a closure cap to adequately seal the contents of a container against leakage from or into the container traditionally has been met by incorporating a soft liner to effect a seal between the under portion of the cap lid and the upper face of the bottle neck rim. The liner may be preformed from sheet or formed in place and is produced from materials or laminar combinations or materials which provide easy cold formability to enable the liner to conform to the individual configuration of the neck rim, including manufacturing aberrations and defects. Because of the specialized sealing function of a liner, it is typically made from softer polymers than those selected to perform the more structural cap functions of providing a strong resilient enclosure for the neck opening with a strong mechanical engagement therewith. In some instances stiffer and stronger polymers, including some which are suitable for producing threaded caps, may be foamed to produce an expanded, less dense sheet having a softer, more flexible characteristic and liners may be made therefrom.
An important characteristic sought for liners and not generally met, especially by plastic caps where the cap lid geometry and dimensions may be affected in time by internal pressure and/or heat exposure, is the ability to adjust to such dimensional changes without undue loss of sealing pressure. This calls for a liner with a high level of resilience and resistance to cold flow, particularly for carbonated and/or pasteurized foods and beverages employing plastic caps, to offset the large amounts of cold flow or creep which can result in a dome shape lid. Most soft, conformable liners by their nature will cold flow to adapt to the initial cap geometry but do not have the elasticity or resilience to adapt to such changing cap geometry and can lose their sealing engagement. An ideal liner, therefore, would possess a soft, easily conformable sealing surface, backed by a resilient supporting structure having good resistance to plastic creep to assure a good sealing engagement at all times under all conditions. Such an ideal liner could be vulcanized rubber which can possess both softness and resiliency over long time periods. However, the cost of such seals precludes their use in most applications. On the other hand, plastics which are suitably soft exhibit poor long term creep resistance and resilience. An alternative approach in popular use is a laminate of a springy paperboard substrate with a soft conformable sealing surface such as wax or plastic. However, this approach has significant performance limitations especially when moisture is present.
In any event, all cap liners add another component to the closure and significantly add to its cost.
Because of an economic advantage, attention has been devoted in recent years to developing caps which have an integral, "linerless" seal. The availability of such semi-rigid plastics as polypropylene and polyethylene, which combine a moderate level of strength and resilience with a moderate level of softness and conformability, has made possible popular use of caps with linerless seals. Typically, such caps employ a circular flange under the cap lid having a wedge shape cross section the lower corner of which is thin and flexible and intended to abut the top surface of the bottle neck rim in a compressive action for sealing. The wedge shape flange generally is vertical and provides a sealing area restricted to the width of the narrow, more flexible portion of the wedge shape. For their effective use, they depend upon a very high level of sealing force on a very limited sealing area which makes them susceptible to sealing surface imperfections, wide dimensional variations in container necks and the decay of sealing force over long time periods.
Other linerless caps employ conical flanges which present an angular cross section from the vertical so that capping will cause the flanges to flex and slide out over the top surface of the neck rim thereby creating a somewhat larger sealing area than obtainable with vertical flanges in straight compression. While the larger, though still limited sealing area has positive advantages, this is offset by the fact that the sealing pressure is at the same time reduced proportionately to the increase in sealing area and they too perform poorly with container necks having wide dimensional variations. Another important limitation of such conical linerless features is the difficulty of removing such features from an injection mold. This results in more complex and costly mold construction and operation and also excludes the more rigid plastics from use.
Some other linerless caps employ conical flanges which engage the corners of the neck rim with the underside of the flange. Such features rely on the use of very high sealing pressure directed against a restricted line contact at the rim corners to obtain sealing integrity. In such cases, sealing integrity depends on container rim corners which are without blemishes as produced and which, because they are most susceptible to marring during handling, are suitably protected from such before they are capped and sealed. Also, to the extent that the conical flanges approach the shape of a cylinder, their sealing integrity is significantly affected by out-of-round or other common dimensional variations of the container manufacturing process or variations between manufacturers resulting from the fact that inside neck dimensions typically are not specified. And to the extent that the flanges become more conical, more complex and costly mold constructions and operation result.
Still another type of linerless cap employs a plug configuration in sealing contact with the inside wall of the container neck. This type of seal has the advantage of engaging that surface of the bottle neck which may be freest from manufacturing defects and most protected from incidental marring in handling thereafter. However, wide manufacturing dimensional tolerances and the industry-wide practice of not specifying the neck bore dimension impose severe limitations in trying to obtain consistent sealing engagement and integrity. As a result, resistance to tapered plug seals can push the cap lid up to varying degrees of undesirable dome shapes. Or such plug seals can yield unacceptably wide variations in sealing engagement and pressures. Efforts to overcome such deficiencies have led to proposed designs with flanges extending radially from generally cylindrical plugs wherein the outer rim of the flange makes a narrow sealing contact with the neck bore and is supported by a hinged flexing action. (See, for example, U.S. Pat. Nos. 4,090,631, 4,016,996 and 4,210,251). An additional problem has been encountered with this type of linerless seal in that the lip or rim of the flange may be distorted by the neck rim during capping leading to imperfect seals. Efforts to eliminate this problem can introduce other problems specific to pressurized containers wherein blow-off or missiling of the caps can occur during uncapping. Another effort to avoid distortion of the lip or rim of such a seal is a cap design and method of producing it wherein a radially extending flange having a downward orientation as molded is hingedly "bent", "folded" or inverted into an upward orientation before it is applied to the container. (See U.S. Pat. No. 4,210,251 and British Patent No. 1,024,762). This is accomplished with extra mold portions and actions during part removal or subsequently in an appropriate fixture to hingedly invert the flange. This effort, therefore, requires the molding of a seal of complex shape utilizing a complicated and costly mold construction and molding operations followed by inverting the sealing portion of the seal hingedly to alter its orientation but not its shape.
Importantly in all cases an inherent limitation to heretofore available linerless caps is that the sealing surface has the same plastic in the same physical state as the structural portion of the cap. This has called for a compromise in the softness and conformability of the sealing surface or in the strength of the structural cap portions, or most frequently both, with consequent limitations in the cap usefulness. That is, to achieve a softer more conformable seal, poorer thread strength must be accepted or to achieve greater thread strength, a harder, less conformable seal must be accepted.
Thus, known caps with linerless seals are beset with drawbacks and problems associated with their need to perform with container necks having imperfect sealing surfaces and wide dimensional tolerances; their limited sealing integrity based on restricted sealing area and loss of sealing pressure over extended periods of time especially at elevated temperatures or with internal pressure or vacuum; the fact that sealing surface softness and conformability are limited; the fact that the use of more rigid plastics are not feasible; and the higher cost and complexity of mold construction and operation for a number of the proposed sealing designs.